Sheath flow is a widely used technique for a variety of applications, including but not limited to particle counting, flow cytometry, waveguiding, and fluid control. Sheath flow involves surrounding a central flow stream (the core) with a surrounding stream (the sheath). In particle counting and flow cytometry applications, the sheath prevents particles in the core from coming into contact with the walls of the channel, thus preventing adhesion and clogging. The sheath also serves to focus the particles or molecules into the center of the channel, allowing for easy counting or measurement through optical or other means. Sheath flow can also be used with fluids of different refractive index to create a waveguide in the core or sheath stream in order to measure transfer of analytes from one stream to the other or to control the rate of interaction between molecules in one or both streams for carefully controlled chemistry or analysis.
Previous designs have created sheath flow through an annular arrangement. A small nozzle was positioned inside a larger tube. The core solution was pumped through the nozzle and the sheath solution was pumped through the larger tube. This configuration required careful alignment of the two tubes and did not easily lend itself to miniaturization. Since the diameter of the nozzle was fixed, the relative sizes of the core stream and sheath solution were relatively constant within a set range.
Other devices provide sheath flow on a chip, but the flow typically operates only in two dimensions. The core stream in these devices is bordered on either side by the sheath streams, however the core is not sheathed top and bottom. The complexity of the support plumbing for these devices is increased, as the number of flow streams is increased from two to three as compared to the annular arrangement design. It is possible to sheath the stream on the top and bottom of the core stream in these systems by adding two additional inlet ports in the top and bottom of the channel. However, this greatly increases the manufacturing complexity of the device. Micromachining technologies are inherently two-dimensional. Three-dimensional channel paths can be created by stacking several two dimensional designs on top of one another, but this adds to the complexity and difficulty of the manufacturing process. Creating a fully sheathed flow in this way could require at least several individual levels, which must be independently produced and then carefully aligned. In addition, use of the device could require multiple pumps to provide solutions to all the inlets.
Flow cytometry is a common technique used to count and evaluate cells and other particles in suspension. In traditional flow cytometers, the sample solution exits a small tube into the center of a larger tube, carrying clean solution. The larger tube is then constricted so that both streams are reduced in diameter and accelerate. The sample stream is reduced in diameter to roughly the size of the cells to be analyzed. This forces the cells to travel in single file, along a fixed and highly precise trajectory within the flow channel. Because the cells are positioned so reproducibly, high numerical aperture optics can be precisely aligned to interrogate them. Alternatively, electronic methods, such as capacitance or impedance changes, could be used for interrogation. Also, because the cells are all following the same path down the channel, they all have the same velocity. This allows the duration and intensity of signals to be correlated with individual cells and particles.
Because of the success of bench-top cytometers, there have been several attempts to create a miniaturized flow cytometer. The laminar flow found in most microfluidic systems makes them at least theoretically well suited to flow cytometry. In practice, however, emulating the annular design of the traditional cytometers is a difficult fabrication problem.
Some flow cytometers operate by simply filling the whole channel with the sample stream. Optical detection can be problematic in these systems because the cells are evenly distributed in the channel. Reducing the dimensions of the channel makes it easier to focus the optics tightly onto the cells, but also increases the risk of clogging. Light scatter off the walls of the channel is also a problem with these systems. Another flow cytometer operates by confining the flow top and bottom between two hydrophobic PDMS layers, and on the sides by air. A variety of factors effect the size of the “channel,” including the hydrostatic pressure and surface tension of the fluid. This system also suffers from the light scattering issues of the previous designs. In addition any contamination of the PDMS surface will change the containment of the solution and may ultimately cause it to fail.
Another flow cytometer system approximates an annular design by focusing the sample stream in one dimension. The sample stream was passed through one arm of a cross intersection while sheath streams are introduced through the two perpendicular arms to laterally constrict the sample stream to the center of the channel. The sample is only confined on the sides, therefore the cells come in contact with the top and bottom of the channel, creating the risk of fouling, and often necessitating the addition of a dynamic coating such as bovine serum albumen, hydroxylpropylmethyl cellulose, or covalent coatings such as trichlorohexadecylsilane. Also, the fact that the cells are distributed vertically means that the optics must have a relatively low numerical aperture, which decreases the amount of light that can be gathered from a single cell and reduces the spatial resolution of the measurements.
Other flow cytometer systems attempt to sheath the sample stream both horizontally and vertically, typically by adding an additional two channels to sheath the stream vertically as well as horizontally. From the standpoint of cytometry, this is a far better situation, because the sample is now completely isolated from the channel walls, and the position of the particles to be analyzed is fixed. Unfortunately, the addition of another set of sheath inputs brings the total number to four. Their relative flow rates must be carefully controlled or the position of the sample stream will drift and the particles will no longer pass through the aligned interrogation region. The best way to ensure even distribution of flow among all the sheath channels is to have a separate pump supplying each stream, but this substantially increases the expense and complexity of the supporting fluidics.
Therefore there is a need in the art for a method and device of providing a sheath flow that fully surrounds the core, can be varied in size, and is easy to manufacture and use for a wide variety of applications.